Add dummy dart:html library for documentation.

lib/html/doc/interface/WebKitNamedFlow.dartdoc

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName WebKitNamedFlow
+interface WebKitNamedFlow {
+
+  /** @domName WebKitNamedFlow.contentNodes */
+  final NodeList contentNodes;
+
+  /** @domName WebKitNamedFlow.name */
+  final String name;
+
+  /** @domName WebKitNamedFlow.overset */
+  final bool overset;
+
+  /** @domName WebKitNamedFlow.getRegionsByContentNode */
+  NodeList getRegionsByContentNode(Node contentNode);
+}